Method of testing the seals of food containers and containers suitable therefor

ABSTRACT

A thin-walled flexible container containing sterilised foodstuff is tested by measuring the electrical conductivity of a path from the interior of the container through the seal to the outside. A container which can be so tested non-destructively is described.

United States Patent [191 Fielibert et a1.

[ Dec. 17, 1974 METHOD OF TESTING THE SEALS OF FOOD CONTAINERS ANDCONTAINERS SUITABLE THEREFOR [75] Inventors: Jozef Frans Fielibert,Zevenaar;

Adrianus Van Rooijen, Didam, both of Netherlands [73] Assignee: LeverBrothers Company, New

York, NY.

221 Filed: June 28,1972

21 Appl. No.: 266,939

30 Foreign Application Priority Data July 1, 1971 Netherlands 7109071 52US. Cl 324/65 R, 53/78, 73/52, 324/65 P 51 Int. Cl GOlr 27/02 [58] Fieldof Search 324/65 R, 65 P; 73/52; 53/78 Primary Examiner-Robert .1.Corcoran Attorney, Agent, or FirmLever Brothers Company [5 7] ABSTRACT Athin-walled flexible container containing sterilised foodstuff is testedby measuring the electrical conductivity of a path from the interior of.the container through the seal to the outside. A container which can beso tested non-destructively is described.

2 Claims, 3 Drawing Figures PATEF-ITED W SHEET 1 BF 2 lrIIIIIIIl/lIl/rPATENTEDBEBTYIBH sumzurg METHOD OF TESTING THE SEALS OF FOOD CONTAINERSAND CONTAINERS SUITABLE THEREFOR This invention relates to the controlof packaging, particularly for foodstuffs in sealed containers, andspecifically to checking the seals of such containers.

Semi-rigid and flexible containers are being used more and more for thepackaging of products, particularly foodstuffs which have been preservedby sterilisation. These semi-rigid or flexible containers, manufacturedfrom materials such as metal (e.g., aluminium) coated on one or bothsides with plastic or optionally made entirely of plastic, give goodprotection as regards tightness with respect to the surroundings as longas no damage occurs, for example as a result of rough handling. However,a source of trouble is provided by the seals for which it is notpossible to guarantee tightness in 100 percent of cases, no matter whatmethod of sealing is used, for example heat-sealing, electric oracoustic welding, etc. It will be realised that if a sterilised foodcontainer has a leak in its seal, the contents rapidly becomecontaminated.

Therefore, during the packaging of foodstuffs according to the abovementioned method, random test ing of the tightness of the seals isregularly carried out by the industry during production. Not only arethese random tests extremely time consuming with the methods used up tonow (e.g., a biological test), but they also generally have thedisadvantage that they must be carried out in a manner which is whollydestructive for the containers which have been filled with the product,closed and possibly thereafter sterilised.

A quick and non-destructive method was sought according to which all thecontainers, if necessary, could be subjected to an effective control ofthe imperviousness to leakage as they leave the production line. We haverealised that measurement of electric resistance across the seal yieldsfavourable and reproducible results. Although electrical resistancetesting is known from the prior art for other purposes, eg, for rubberarticles (Austrian patent specification No. 147,124), it

cannot be used for the purpose aimed at here without the necessaryadjustment.

Accordingly the invention provides a sealed container containing aproduct and incorporating an'electrode which provides an electricallyconductive path between the product and the exterior of the container,said electrode being electrically insulated in such a manner as toenable conductivity between the product and the exterior of thecontainer to be measured when at least the seal of the container isimmersed in an electrolyte.

The conductivity (or resistance) can be measured in conventional mannerby measurement of the current flow produced by a constant potentialbetween the electrolyte and electrode while the container is suitablyimmersed, and this provides a measure of the effectiveness of the seal.

It should be realised that the invention is only applicable to themeasurement of containers containing a product which is electricallyconductive. This applies to most sterilised foodstuffs.

The electrode may be incorporated in the container permanently duringformation of the container or during the sealing of its lid, or may beonly temporarily incorporated during the testing operation.

A particularly convenient form of the former makes use of a strip offoil, or wire which passes through the container seal, i.e., it issandwiched between the mating sealed surfaces at the seal, from outsidethe container to the product inside.

Desirably the wire or strip at points outside the container should havean insulating coating to prevent risk of direct electrical contactbetween the wire or strip and the electrolyte, and there should be noelectrical contact with the walls of the container if the latterincludes a layer of metal foil.

A method according to the invention comprises incorporating an electrodein said container which provides an electrically conductive path betweenthe product and the exterior of the container immersing the container inan electrolyte, making an electrical connection to the exterior of saidelectrode while maintaining electrical insulation between said electrodeand the electrolyte, measuring the conductivity between said electricalconnection and the electrolyte, and removing the container from saidelectrolyte and from the electrical connection.

In a particularly convenient form where the electrode is onlytemporarily incorporated in the container, said electrode is aconductive needle which is injected through a point in the walls of thecontainer and is subsequently removed, and the method further comprisesresealing thev container at said point in its walls.

A convenient apparatus for carrying out this form of the processaccording to the invention incorporates both a conductive needleelectrode and means for provision of a drop of molten plastic to re-sealthe hole formed by the needle.

This latter method is particularly suitable for containers manufacturedentirely from plastic.

In certain cases however this method could also be useful forsemi-rigids made of metal. It is then neces sary though that, at leastduring the measuring, the product cannot come into contact with the cutedge caused in the metal by the conductive probe needle, oralternatively that the needle is insulated from the metal.

The device for carrying out the method according to the invention isprovided with a bath filled with electroconductive and satisfactorilymoistening liquid (electrolyte) in which the containers, and at leastthe part of them that is to be controlled, can be partially or whollyimmersed, and further with a measuring device comprising a voltagesource, a micro-ammeter and two electrodes, one of which sticks into theelectrolyte and one of which is intended for contact with the productpresent in the container to be immersed in the bath. The secondelectrode may simply be the wall of the bath if it is electricallyconductive.

For this we preferably make use of containers provided with a conductorwhich is electrically insulated from the wall of the container. This canbe provided in the wall previously or can be inserted right through theseal during the sealing operation.

In another embodiment the electrode intended for contact with theproduct consists, according to the invention, of a conductive probeneedle which is provided with a driving device for limited longitudinalmovement of the needle between a measuring position and a withdrawnresting position, while means are present for dosing a measured amountof liquid plastic after each measuring operation. Except for the part atand near the extremity, the conductive probe needle can be provided withan insulating coating.

Containers which have been controlled according to the methods indicatedare easy to identify either because of the plugging of the control holein their bottom wall or because of the presence of an electrode.

The invention will be further explained with reference to the drawing.In this:

FIG. 1 is a sketch showing the principle involved in carrying out ameasuring on a container provided with a plate electrode;

FIG. 2 shows, on an enlarged scale, the arrangement with a containerprovided with a tape electrode;

FIG. 3 shows schematically an apparatus for measuring the imperviousnessto leakage, more particularly intended for containers made entirely fromplastic.

In the figures an electrolyte bath is indicated by 1, containers to beinvestigated by 2 and a product present in the containers by 3.

In FIG. 1 is represented schematically a measuring operation on acontainer which has previously been provided with a plate electrodewhich at 4 on the bottom, electrically isolated from the metal outerside of the container, can be connected to the measuring system. Thelatter is only very schematically illustrated in the drawing andconsists of a voltage source and a ,u-ammeter connected to it in series.

In FIG. 2 a more easily realised tape or wire electrode 5 is shown whichcan be welded within the seal when the latter is being affected. Thiscan be carried out with standard containers in the production line,irrespective of the kind of container. In FIG 2 a container with anoutside of metal (e.g., aluminium) is shown; the tape or wire electrodecan, however, also be applied to containers made entirely of plastic.The electrode carries an external insulating coating to provideelectrical insulation from the electrolyte.

Finally, in FIG. 3 an apparatus is shown which is preferably intendedfor measuring the imperviousness to leakage of containers made entirelyof plastic, e.g., polypropene. Herein a conductive probe needle 6 iscarried by a driving device 7 and moved manually from a withdrawnresting position to the measuring position, as shown in the figure. Indoing so, the bottom wall of the container 2 is pierced and the needleis caused to extend into the conductive product. The conductivitymeasurement is then made.

On the upper side of the needle, a piston 8 is attached which, when theneedle is subsequently withdrawn, is forced down by the driving device 7(a form of screw jack) and pushes in front of it a measured amount ofliquid plastic, in the present case also polypropene, which is presentin the reservoir 9. The plastic is kept at the right temperature by theheating spiral 10 which surrounds the reservoir 9.

The liquid plastic flows on to the bottom wall of the container, wherebythe hole that has been made in the bottom is closed.

As has already been mentioned in the introduction to the description, itis possible to use the aforementioned apparatus for metallic semi-rigidsbut only, however, if the cut edge is sufficiently insulatedelectrically from the product and/or the needle.

What is claimed is:

1. A method of testing the effectiveness of the seal of a free-standingthin-walled container containing therein an electrically conductive foodproduct said container providing an electrically insulated barrierbetween the food product and the exterior of the container comprisingthe steps of inserting an electrode in a wall of the container, theelectrode being in electrical contact at one portion thereof with thefood product and in electrical contact at another portion thereof with apoint outside of the container while being electrically insulated fromthe container immersing the container in an electroltye, connecting theportion of the electrode outside of the container with a source ofelectrical current while maintaining electrical insulation between saidelectrode and the electrolyte, removing the container from theelectrolyte and removing the electrode. from the wall of the container.

2. A method according to claim 1 in which said electrode is a conductiveneedle which is injected through a point in the walls of the containerand is subsequently removed, and the method further comprises rescalingthe container at said point in its walls.

1. A method of testing the effectiveness of the seal of a freestandingthin-walled container containing therein an electrically conductive foodproduct said container providing an electrically insulated barrierbetween the food product and the exterior of the container comprisingthe steps of inserting an electrode in a wall of the container, theelectrode being in electrical contact at one portion thereof with thefood product and in electrical contact at another portion thereof with apoint outside of the container while being electrically insulated fromthe container immersing the container in an electroltye, connecting theportion of the electrode outside of the container with a source ofelectrical current while maintaining electrical insulation between saidelectrode and the electrolyte, removing the container from theelectrolyte and removing the electrode from the wall of the container.2. A method according to claim 1 in which said electrode is a conductiveneedle which is injected through a point in the walls of the containerand is subsequently removed, and the method further comprises resealingthe container at said point in its wAlls.